Immunities are procedural impediments to the exercise of jurisdiction by courts, ordinarily domestic courts. It is a mainstay of international law that states are entitled to immunity under customary international law (see for an attempt at codification: United Nations Convention on Jurisdictional Immunities of States and Their Property (UN Convention Jurisdictional Immunities).1 This immunity finds its justification in the principle of the sovereign equality of states, according to which no state can sit on judgment of another state, which after all is formally its peer under international law.
Immunities have also been accorded to international organizations. This has happened on the basis of constituent documents rather than customary international law or general principles of law (League of Arab States, Ryngaert).  The rationale for such immunities also differs: they do not serve to protect international organizations from interference by their peers—other international organizations—but rather by the domestic courts of states, including their member states. Therefore, international organization immunity is essentially geared to protecting the autonomy of the international organization vis-a-vis states.
As immunity is an issue that plays out in domestic courts, there is a wealth of domestic decisions on the issue. Moreover, there is a limited number of international decisions ruling on the immunity determinations made by domestic courts, in particular those of the European Court of Human Rights (ECtHR), but also of the International Court of Justice (ICJ).
Formally speaking, the immunity of international organizations has never been considered as absolute, unlike the immunity of states. Only insofar as international organizations act within the powers conferred on them by the member states, can they rely on immunities. In practice, however, this ‘functional’ immunity has become quasi-absolute, as international organizations do not normally act ultra vires. However, two movements in the case-law have tended to erode this immunity: the importation of restrictive immunity from the law of state immunity, and the application of human rights, the right of access to a court in particular. Obviously, international organizations could also decide to waive their immunity.
The first movement can mainly be associated with US courts, v of which have ruled that there is a commercial exception to the immunity of international organizations similar to the exception to state immunity featuring in the US Foreign Sovereign Immunities Act (Nokalva, Boon). Not all US courts have been taking this path, however, and have affirmed the international organizations’ quasi-absolute immunity (see, e.g. Askir, Ruys).
The second movement, inspired by procedural human rights, can be associated with Europe, but again, its application is not uniform. As early as the 1960s, claimants began to advance the argument that international organizations should offer them a right to a remedy, in the absence of which they should be allowed to sue the international organization in domestic courts. This argument was initially dismissed and the quasi-absolute immunity of international organizations was confirmed; it was even recognized that international organizations such as the UN did not offer proper remedies via internal mechanisms (Manderlier, Schmitt). In 1999, however, in Waite and Kennedy and Beer and Regan (note Neumann and Peters), the ECtHR ruled, regarding an employment case brought against the European Space Agency (ESA) before German courts, that ‘a material factor in determining whether granting ESA immunity from German jurisdiction is permissible under the Convention is whether the applicants had available to them reasonable alternative means to protect effectively their rights under the Convention.’ After these decisions, it remained unclear, however, whether contracting states to the European Convention on Human Rights (ECHR) were required to dismiss an international organization immunity claim in case the relevant international organization did not make reasonable alternative means available to guarantee a claimant’s right to a remedy. Some Belgian courts have turned out to be the strictest, by going as far as to reject an international organizations’ immunity in case its dispute-settlement mechanism did not meet the quality guarantees required by art. 6 ECHR, which enshrines the right to a remedy (Siedler, Schmitt). In this approach, the mere existence of a dispute-settlement mechanism does not suffice for an international organization to plead its immunity before domestic courts. Most courts have not interpreted the Waite and Kennedy principle too strictly, however. This deferential approach to the international organization may well be in keeping with what the ECtHR meant, as also borne out by its follow-up judgment in Chapman v Belgium (2013) ECHR 2013, whereby the NATO Appeals Board appeared to satisfy itself with a level of internal protection that is equivalent rather than identical to the protection offered by art. 6 ECHR. Some courts have even taken the view that art. 6 ECHR does not apply where international law limits the scope of jurisdiction of a domestic court (Entico, Wickremasinghe), although such a view is in tension with the Waite and Kennedy principle. Still, the ECtHR itself has held that Waite and Kennedy does not apply in procedures against the UN in respect of peace operations. According to the Court, given the UN’s special status as a collective security organization, its quasi-absolute functional immunity cannot be abrogated by human rights considerations (Mothers of Srebrenica, Momirov).
Aside from case-law on the immunities of international organizations as such, there is some case-1 aw on immunities of international organization officials. In principle, international organization officials, like state agents, enjoy immunity ratione materiae for acts performed in an official capacity, as deciding otherwise would allow courts to circumvent the immunity of the organization. Some higher-ranking international organization officials also enjoy immunity ratione personae during their term of office, for acts performed in both an official and a private capacity, just like diplomats or some high-ranking state officials.
It may not always be clear whether an international organizations’ official acts in an official or a private capacity. A thorny question in this respect is whether the domestic court hearing the case against the official, or rather the international organization can make this determination. The leading ICJ case on the issue is the Advisory Opinion in Cumaraswamy (officially known as Difference relating to Immunity from Legal Process of a Special Rapporteur of the Commission on Human Rights, Wickremasinghe). In this opinion, the Court did justice to the demands of both the UN and the member state hearing the case, by entitling the UN’s view on the immunity of its officials to ‘a presumption, which can only be set aside by a national court for the most compelling reasons and is thus to be given the greatest weight by the national courts’. Cumaraswamy also constituted the first ‘binding advisory opinion’ of the Court in the sense of art. VIII Section 30 of the 1946 UN Convention on Privileges and Immunities, which stipulates that an advisory opinion given by the Court on a difference arising out of the interpretation or application of the Convention will be accepted as binding by the parties.
A final issue that is worth mentioning in an immunity context concerns the availability of service of process regarding international organizations. Domestic legal systems often require that the defendant is provided with actual notice of a suit. This process is not self-evident when it comes to serving international organizations: in many states consent of the international organizations’ Head is required. Where consent is not secured, the international organization is formally not put on notice, and cannot appear in court (Prewitt, Boon). In such cases, the immunity question will not even be addressed by the court.
-  Adopted 2 December 2004, opened for signature 17 January 2005 until 17 January 2007, UN Doc.A/RES/59/38.
-  But see Supreme Court 20 December 1985, LJN AC9158 (AS v Iran-United States Claims Tribunal)(conferring immunities on the Iran-US Claims Tribunal, considered as an international organization, onthe basis of customary international law).
-  See, for example, art. 105(1) Charter of the United Nations (adopted 26 June 1945, entered into force24 October 1945) 1 UNTS XVI (UN Charter): ‘The Organization shall enjoy in the territory of each of itsMembers such privileges and immunities as are necessary for the fulfilment of its purposes.’
-  In Askir, the Court found it unnecessary to decide whether the restrictive immunity doctrineapplied; it eventually applied the functional immunity doctrine and granted immunity to the UN. Askirv Boutros-Ghali, 933 F. Supp. 368 (S.D.N.Y 1996).
-  cf. Waite and Kennedy v Germany (1999) ECHR 1999-I 393, para. 68.
-  See the Convention on the Privileges and Immunities of the United Nations (adopted 13 February1946) 90 UNTS 327, art. V. and art. 105(2) UN Charter.
-  cf. Difference Relating to Immunity from Legal Process of a Special Rapporteur of the Commission onHuman Rights, Advisory Opinion,  ICJ Rep 62, para. 61.